The present invention relates to a power amplifier apparatus having an over-current protecting function for protecting a load or output transistor from being destroyed by an excessive output current, and more particularly to improvements in effective protection for a load and output transistor in an power amplifier apparatus in which a load is connected between output terminals of two power amplifiers for complementarily effecting amplification of a signal.
A so-called BTL (Balanced Transformerless) system has been known, as disclosed in, for example, FIG. 7 of U.S. Pat. No. 3,376,388, in which a pair of direct-coupled power amplifiers of an OTL (output transformerless) system are used and operated so as to respond to one input signal for respectively producing outputs of opposite phases, and the outputs are fed to a load connected between output terminals of the pair of power amplifiers. In such a power amplifier apparatus of the BTL system, if an output terminal of one power amplifier should contact with one end of a power supply to form a short-circuit therebetween, an infinite current would flow through one of output transistors of the power amplifier, and thus the output transistors would be immediately destroyed. The conventional OTL power amplifier normally has a circuit part for protecting the output transistors from being destroyed by an excessive current. The circuit part detects the excessive current of an emitter current in the output transistors and then limits an increase of the base currents of the output transistors so as to suppress the increase in the emitter current or to decrease the emitter current, and hence the above-mentioned breakdown of the output transistors does not occur. However, in the BTL system, since the power amplifier on the side of the unshort-circuited output is operating normally, a large D.C. current flows constantly through a speaker connected to the power amplifier as a load, and consequently, the voice coil of the speaker would be burnt out. If a D.C. blocking capacitor is inserted between an output terminal of the power amplifier and the speaker, then the burning of the voice coil caused by the aforementioned D.C. current will not occur. However, a large value of electric charge is stored in the D.C. blocking capacitor upon output short-circuiting. Therefore, when the output short-circuiting has been released, the stored charge will discharge through the voice coil, and burn it.